1. Field of the Invention
This invention relates to a process for the production of dimethylformamide. More particularly, it is concerned with a process for producing dimethylformamide which comprises reacting methanol and ammonia to form methylamines, separating a mixture of dimethylamine and trimethylamine from the methylamines, and reacting the mixture of dimethyamine and trimethylamine with carbon monoxide to produce dimethylformamide.
2. Description of the Prior Art
Dimethylformamide (hereinafter abbreviated "DMF") is widely used in many industrial applications, for example, as a solvent for artificial leather, urethane fibers, acryl fibers and various resins, and moreover, as a reaction solvent or reagent for a wide variety of organic syntheses.
For the production of such industrially useful DMF, there are two typical processes. One is based on the reaction of dimethylamine (hereinafter "DMA") and carbon monoxide, and the other is based on the reaction of DMA and methyl formate. In the latter process, however, methanol is an inevitable by-product and the reaction steps are complicated. Therefore, the former process is advantageously employed from the industrial standpoint.
In the production of DMF by the reaction of DMA and carbon monoxide, the starting material DMA should be of high purity for the reasons explained hereinafter.
The reaction of DMA and carbon monoxide is carried out in the presence of an alkali metal alcoholate catalyst. If DMA contains water, the alkali metal alcoholate reacts with the water and it is unfavorably consumed. As a result, the reaction of DMA and carbon monoxide does not proceed.
In addition, if DMA contains ammonia and other amines, various amides are by-produced during the synthesis of DMF. These by-produced amides are formamide, N-methyl formamide, N,N-dimethylacetamide and the like. Although their boiling points are higher than that of DMF, their stabilities to heat and water are inferior to that of DMF. In the purification of DMF, therefore, these amides are subject to thermal decomposition in the reboiler of a distillation column and so on, or to hydrolysis in the presence of a small amount of water. As a result, the decomposition products of these amides, i.e., ammonia and amines, formic acid, and the salts thereof are brought into the purification step of DMF. Thus, the purification step is made complicated.
DMF is required to be of high quality from the point of application. That is, it is required not only to be of high purity, but also to meet certain physical requirements. For example, when DMF is used as a polymerization solvent for the production of a polyurethane based artificial leather and synthetic fibers, the addition reaction rate of an active hydrogen compound, such as a polyol, with isocyanate is influenced by the quality of DMF. The addition reaction rate is high when the pH of DMF is high (pH=9 or more) according to the quantity of amines contained in DMF, whereas when DMF contains formic acid and so on, and its pH is low (pH=6 or less), the addition reaction rate is low.
Moreover, when DMF contains organic acid salts, the electric conductivity of DMF increases.
As described above, the physical properties of DMF are greatly influenced by the impurities contained therein. In practical use, therefore, DMF should be of high purity and high stability, that is, contain minimum amounts of impurities. In particular DMF should have an electric conductivity of not more than 1.0 .mu. /cm and the pH of a 20% aqueous solution should be about 6.5 to 8.5.
For the above reasons, in producing DMF by reacting DMA and carbon monoxide, the use of DMA containing unreacted ammonia as a feed, other methylamines, water and so on has been avoided, and high purity DMA has been used as a feed. Hitherto, high purity DMA has been obtained by reacting methanol and ammonia to provide methylamines, isolating DMA from the methylamines, and purifying the DMA. In the reaction of methanol and ammonia, monomethylamine and trimethylamine are inevitably by-produced in addition to DMA as a result of the equilibrium. Among these methylamines, it is DMA that is mainly used in industrial applications. Therefore, surplus monomethylamine and trimethylamine are recycled in the synthesis of methylamines. This leads to installation of a larger scale apparatus for production of DMA which is not in proportion to the production amount of DMA.
In separating DMA from the by-product methylamines, distillation is generally employed. Since these three methylamines, i.e., monomethylamine, DMA, and trimethylamine, and unreacted ammonia form azeotropic mixtures among them, multi-stage distillation is carried out in isolating DMA from the reaction product methylamines.
A typical example of the methods of synthesizing methylamines which are commercially conducted, is described in Fluid Handling, January (1963), pp. 13 to 14. According to this method, the reaction product obtained by the catalytic reaction of methanol and ammonia is fed into a first distillation column wherein the unreacted ammonia is withdrawn from the top of the column and recycled to the reaction system. The effluent from the bottom of the column is fed into a second distillation column wherein trimethylamine is isolated from the top of the column by water-extraction distillation, and from the bottom of the column a mixture of DMA, monomethylamine and water is withdrawn. This mixture is fed into a third distillation column wherein the mixture is subjected to dehydration treatment and a mixture of DMA and monomethylamine is withdrawn from the top of the column. The DMA-monomethylamine mixture is introduced into a fourth distillation column wherein the mixture is subjected to an ordinary distillation treatment, monomethylamine is isolated from the top of the column, and DMA is withdrawn from the bottom. The thus obtained high purity DMA is used as a starting material for production of DMF wherein DMA is reacted with carbon monoxide in the presence of an alkali metal alcoholate catalyst such as sodium methylate and the reaction product is distilled and purified to give DMF.
In the prior art DMF production processes as described above, however, the production of DMA is complicated, the operation is troublesome, large quantities of utilities such as steam are consumed, and thus there are many industrial problems which need to be solved.